DESCRIPTION The research proposed desires to understand the events occurring within the lipid bilayer when the HA protein of influenza virus mediates membrane fusion. Fluorescence resonance energy transfer (FRET), polarization anisotropy and patch clamp measurements will be used to monitor changes in both viral and target membranes during the fusion process. Changes will be monitored using a set of fluorescent probes that have been synthesized and developed by the foreign collaborators. The specific aims are: 1. Ganglioside clustering during virus binding and membrane fusion that is triggered by lowering pH will be studied. 2. The distances between virus and liposome membranes in the prefusion state will be evaluated. 3. The role of ganglioside-bound HA and unbound HAs in causing fusion will be determined. 4. The kinetics of mixing of the inner and outer lipid monolayers during virus-liposome fusion will be compared. 5. The influence of the lipid composition in liposomes on the kinetics of lipid mixing will be studied. 6. The kinetics of pore growth between HA-expressing cells and large unilamellar vesicles will be measured.